CrazyC83 wrote:My estimate for the peak intensities for each storm (Alberto and Chris are TCR confirmed):
Helene - 125 kt / 938 mb
* Winds based on 136 kt at 700 mb (122 kt at surface) and radar data supporting about 160 kt at 850 mb (128 kt at surface; the SFMR is ignored due to shallow water). Pressure appears reasonable based on data just inland. Peak intensity was at Florida landed.
Hi Crazy, I appreciate your analysis and the time you invest in putting together your assessments.
As far as Helene is concerned, I’m confident that it weakened prior to landfall and was more than likely a 110 kt Cat 3 as opposed to the operationally assessed 120 kt Cat 4.
First, I can’t find the VDM containing the 136 kt FLW mentioned in the 11 pm EDT advisory by the NHC. Thus, I’m not sure the actual time that specific measurement was obtained? If it was taken within the last 30 minutes before landfall, we can use it to determine the intensity estimate. If not, the latest one I could locate was 129 kt at 931 pm EDT.
Regardless, there’s no doubt in my mind that the standard 90% rule wasn’t applicable in this particular case. The convection had significantly weakened in the SE quadrant within the last hour or two preceding landfall. As such, I’d use a possibly generous 85% factor. That would equate to an intensity estimate of either 115 or 110 kt.
I’d really like to see what the WL150 readings were to get a better idea of how well the FLWs were translating to the surface. Again, it certainly wasn’t as efficient as usual. That was most obvious on the ground in both actual wind measurements, as well as the structural and tree damage that was arguably closer to a Cat 2 than Cat 4. This was also supported by the relatively lackluster SFMR readings, that although pretty untrustworthy, they are generally inflated by shoaling…not the other way around.
The actual barometric pressure observations very near the point of landfall clearly show Helene was on the decline by the time it crossed the coastline at 1110 pm EDT. To be specific, a lowest observed pressure of 942.8 mb was recorded just 6 n mi from the landfall point and a whole 2 minutes after landfall occurred. Based on this in-situ data, the lowest possible pressure one could reasonably argue would be 941mb.
The observed wind speed measurements just inland of the coastline were pretty pedestrian for a supposed Cat 4 hurricane…especially one moving at such an anomalously fast translational speed, no less.
Lastly, the infrared satellite imagery shows a markedly degrading appearance on final approach to landfall…with warming CDO convection and filling of the eye. Similarly, the radar presentation also matched this significant weakening in the SE quadrant where the highest FLW was observed.
Taken altogether, it’s fairly obvious that Helene weakened rather substantially in the last 1 to 2 hours prior to landfall and didn’t possess the standard 10% FLW to surface ratio, as a result of weakening and relatively sparse convection in the SE eyewall that inhibited an efficient transport of those winds to the ground. At most, Helene was very likely no stronger than 115 kt/941 mb at landfall, although I’d lean to 110 kt, myself.
https://www.weather.gov/source/tae/Trop ... essure.csv
what a crazy season.
